Crystal structure of the axial (orange

Znorg. Chem. 1982, 21, 1318-1321
1318
to the least-squares plane of the acac ring.
Bond distance between the CY and P carbons of the naphthalene ring are shorter than the P-P bonds observed in other
naphthalene derivatives. lo The naphthalene ring inclines
slightly as the benzo group approaches N(2), and their dihedral
angle is 87.6 (1)' to the least-squares plane of the acac ring.
Since the inclination of the aromatic ring results in only a slight
change in the direction of polarization, it seems to contribute
(1)O
(10) Harata, K.; Tanaka, J. Bull. Chem. SOC.Jpn. 1973,46, 2747. Ferraris,
G.; John, D. W.; Yerkess, J.; Bartle, K. D. J . Chem. Soc., Perkin Trans.
2 1972, 1628 and literature cited in these references.
little to the rotational strength of these complexes in the present
series6
Acknowledgment. This research was supported in part by
a Scientific Research Grant from the Ministry of Education
to which the authors' thanks are due.
Registry No. I.2BF4.2H20, 80327-54-4.
Supplementary Material Available: A listing of observed and
calculated structure factors and tables of thermal parameters (Table
111), observation of Bijvoet pairs (Table IV), possible hydrogen bonds
(Table V), interatomic distances outside of the molecule (Table VI),
nonessential bond lengths and angles (Table VIII) (24 pages). Ordering information is given on any current masthead page.
Contribution from the Department of Chemistry,
University of California, Berkeley, California 94720
Crystal Structure of the Axial (Orange-Yellow) Isomer of
Bis( methyldiphenylphosphine)tetrakis( trifluoroacetato)dimolybdenum
GREGORY S. GIROLAMI and RICHARD A. ANDERSEN*
Received June 17, 1981
The X-ray crystal structure of the orange-yellow isomer of Mo,(O,CCF,)~(PM~P~,)~
confirms that it possesses a class
I (axial) structure as previously deduced spectroscopically. Accordingly, PMePh, is capable of forming both class I and
class I1 (equatorial) adducts with Mo2(O2CCF,),. NMR studies show that an equilibrium between the two isomers is
established in solution. Crystal data: space group P2,/c; a = 9.923 (1) A, b = 22.517 (2) A, c = 18.735 (3) A, = 99.18
(1)'; V = 4132 (2) A3;Z = 4; R = 4.43%, R, = 6.63%; Mo-Mo = 2.128 (1) A, Mo-P = 2.988 i 0.024 A, Mc-Mc-P
= 166.15 A 0.16O.
W e have recently shown that tetrakis(trifluoroacetat0)dimolybdenum yields 2: 1 coordination complexes, Mo2(02CCF3)4(PR3)2,with various tertiary phosphines.' Two
general classes of complexes were isolated, referred to as class
I and class 11, as judged by infrared and N M R spectroscopy.
Class I complexes were those in which the phosphine ligands
were coordinated to axial sites (A),while class I1 complexes
FF3
FF3
Table I. Crystal Data
V = 4132 (2) A 3
spacegoup: P2,/c
z=4
Q = 9.923 (1) A
b = 22.517 (2) A
mol wt = 1044.38
c = 18.735 (3) A
dcalcd = 1.679 g cme3
8 = 99.18 (1)"
Lfcalcd 7.8 cm-'
01 = y = 90"
slze = 0.27 x 0.31 x 0.45 mm
diffractometer : Enraf-Nonius CAD-4
radiation: Mo KZ, X = 0.710 73 A
monochromator: highly oriented graphite
scan range, type: 3 Q 28 4 45", e-2e
scan speed, width: 0.6-6.7" min-I, A0 = (0.6 + 0 347 tan e)"
rflctns: 5926, 5393 unique, 4273 with I > 3 4
R = 4.43%
R , = 6.63%
CF3
A
CF3
B
variables = 523
GOF = 3.01
Cotton and Lay were unable to prepare this compound by our
published method and instead synthesized a compound of the
same stoichiometry by a different route. Its color was reported
to be red-orange, and crystallographic analysis revealed a class
I1 (equatorial) structure.2
In view of this discrepancy, we have prepared Mo2(02CCF3)4(PMePh2)2as originally described, with the exception that the solution volume was ca. 5 mL rather than ca.
10 mL. Herein we report its X-ray crystal structure and show
that it is indeed a class I, axial diadduct. Some spectroscopic
comparisons and preparative details involving the orangeyellow and red-orange isomers are also described.
were those in which the phosphine ligands were coordinated
to equatorial sites, one isomer of which is shown (B). On the
basis of a cone angle vs. basicity graph, we showed that class
I1 complexes were formed only by the smallest and most basic
phosphines.'
A subsequent article has demonstrated that the crystal
structures of M o ~ ( O ~ C C F , ) , ( P P ~and
~ ) ~Mo2(02CCF3),(PEt,Ph)2 are in complete accord with our findings, viz., that
the former is a class I complex while the latter is a class I1
complex.2 The structure of the latter is a testament to the
Results and Discussion
predictive value of our cone angle and basicity criteria, since
An ORTEP3 drawing of the orange-yellow isomer of Mo2M O ~ ( O ~ C C F ~ ) ~ ( had
P E ~not~ Pbeen
~ ) ~previously prepared
(02CCF3)4(PMePh2)2
(Figure 1) is that of a class I structure,
by us nor structurally characterized by spectroscopic studies.
as deduced previously by spectroscopic analysis.' Bond lengths
In our original publication, we described the preparation
of an orange-yellow complex, M O ~ ( O ~ C C F , ) , ( P M ~ P ~ ~ )and
~ , angles involving the molybdenum atoms are given in Table
111. Most of the molecular features are quite normal, except
which was assigned a class I (axial) structure.' However,
for the large anisotropic thermal motion of the fluorine atoms
Girolami, G . S.; Maim, V. V.; Andersen, R. A. Inorg. Chem. 1980, 19,
805-810.
(2) Cotton, F. A.; Lay, D. G. Inorg. Chem. 1981, 20, 935-940.
(1)
0020-1669/82/ 1321-1318$01.25/0
(3)
Johnson, C. K. Report ORNL-3794;Oak Ridge National Laboratory:
Oak Ridge, TN, 1965.
0 1982 American Chemical Society
Inorganic Chemistry, Vol. 21, No. 4, 1982 1319
Axial Isomer of M O ~ ( O ~ C C F ~ ) ~ ( P M ~ P ~ ~ ) ~
Table 11. Positional Parameters and Their Estimated Standard Deviations
atom
X
Y
z
atom
X
Y
Z
Mo 1
Mo 2
P1
P2
F1
F2
F3
F4
F5
F6
F7
F8
F9
F10
F11
F12
01
02
03
04
05
06
07
08
0.26101 (5)
0.27183 (5)
0.1871 (1)
0.3194 (2)
0.7499 15)
0.6974 (6)
0.7226 (5)
-0.0030 (4)
0.1887 (5)
0.1397 (6)
-0.1352 (6)
-0.2007 (5)
-0.2075 (5)
0.5300 ( 5 )
0.4033 (7)
0.3460 (6)
0.4658 (4)
0.4790 (4)
0.1938 (4)
0.2050 (4)
0.0565 (4)
0.0698 (4)
0.3278 14)
0.3401 (4)
0.5277 (6)
0.6739 (7)
0.1807 (6)
0.1244 (7)
0.0096 1‘6)
0.47374 (2)
0.55784 (2j
0.36898 (7)
0.65951 (7)
0.4944 (3)
0.5140 (3)
0.5843 (2)
0.5900 (3)
0.5829 (3)
0.6568 (2)
0.4851 (4)
0.4533 (3)
0.5437 (3)
0.4374 (3)
0.3734 (2)
0.4416 (3)
0.4808 (2)
0.5682 (2)
0.5157 (2)
0.6039 12)
0.4624 (2)
0.5516 (2)
0.4284 (2)
0.5151 (2)
0.5272 (3)
0.5323 (3)
0.5722 (3)
0.5995 (3)
0.5049 (3)
0.27584 13)
0.22528 (3j
0.35724 (8)
0.12784 (9)
0.3200 (4)
0.4143 (3)
0.3506 (4)
0.4208 (3)
0.4866 (2)
0.4259 (3)
0.0754 (3)
0.1679 (4)
0.1444 (4)
0.0807 (3)
0.0814 (3)
0.0200 (2)
0.3273 (2)
0.2752 (2)
0.3654 (2)
0.3116 (2)
0.2246 (2)
0.1726 (2)
0.1905 (2)
0.1362 (2)
0.3142 (3)
0.3495 (4)
0.3617 (3)
0.4264 (3)
0.1838 (3)
C6
c7
C8
c11
c12
C13
C14
C15
C16
C17
C18
C19
CllO
-0.1358 (8)
0.3534 (6)
0.4080 (6)
0.1566 (5)
0.0327 (6)
0.0176 (8)
0.1234 (7)
0.2479 (8)
0.2686 (7)
0.2977 (6)
0.4036 (7)
0.4896 (9)
0.4736 (10)
0.3799 (14)
0.2804 (13)
0.0237 (6)
0.1977 (6)
0.2354 (9)
0.1335 (10)
-0.0002 (10)
-0.0464 (10)
0.0609 (8)
0.3354 (5)
0.3297 (7)
0.3468 (7)
0.3675 (7)
0.3713 (7)
0.3558 (6)
0.4856 (7)
0.4975 (4)
0.4606 (3j
0.4306 (3)
0.2994 (3)
0.2727 (3)
0.2208 (3)
0.1942 (4)
0.2216 (4)
0.2755 (4)
0.3462 (3)
0.3830 (4)
0.3665 (6)
0.3153 (5)
0.2786 (5)
0.2951 (4)
0.3845 (3)
0.6680 (3)
0.6775 (5)
0.6830 (6)
0.6778 (5)
0.6668 (5)
0.6629 (4)
0.7345 (3)
0.7854 (3)
0.8413 (3)
0.8475 (3)
0.7970 (3)
0.7402 (3)
0.6501 (4)
0.1432 ( 5 )
0.1404 (3j
0.0787 (3)
0.3083 (3)
0.2930 (4)
0.2519 (5)
0.2294 (4)
0.2427 (4)
0.2801 (4)
0.4396 (3)
0.4667 (4)
0.5313 (4)
0.5651 (4)
0.5364 (5)
0.4751 (5)
0.3871 (4)
0.0459 (3)
-0.0231 (4)
-0.0842 (5)
-0.0783 (5)
-0.0072 (5)
0.0524 (4)
0.1669 (3)
0.1210 (3)
0.1555 (4)
0.2280 (4)
0.2716 (4)
0.2389 (3)
0.0973 (4)
c1
c2
c3
c4
c5
Clll
c112
C113
c21
c22
C23
C24
C25
C26
C27
C28
C29
c210
c211
c212
C213
Table 111. Principal Bond Distances and Angles for Axial Mo,(O,CCF,),(PMePh,),
Dist, A
Mol-Mo2
Mol-P1
M02-P2
Mo2-Mo 1-P1
MO1-MO 2-P2
01-MO 1-05
03-Mo 1-07
02-Mo2-06
04-Mo 2-0 8
2.128 (1)
2.964 (1)
3.012 (1)
av 2.988
166.31 (2)
165.99 (2)
av 166.15
177.35 (10)
177.14 (10)
176.90 (10)
177.53 (10)
av 177.23
Mol-01
-03
-05
-07
2.111 (2)
2.124 (2j
2.117 (3)
2.092 (2)
av 2.111
Angles, Dea01-Mo 1-03
90.36 (9)
-07
89.23 (9)
03-Mol-05
90.73 (10)
05-Mol-07
89.56 (10)
02-M02-04
90.52 (9)
-08
89.32 (10)
04-Mo2-06
90.02 (10)
06-Mo2-08
90.01 (10)
av 89.97
and some of the carbon atoms of the phenyl groups.
The molecular geometry is very similar to that found for
~
that the axial
M O ~ ( O ~ C C F ~ ) ~py( ~=~ ~) y, ,r i d i n e ,except
ligands are substantially farther away from the molybdenum
atoms in the phosphine com lex. The difference, Mo-P =
2.988 A vs. Mo-N = 2.548 , can be ascribed either to the
larger tetrahedral covalent radius of phosphorus (1.10 A)
relative to nitrogen (0.70 A)5 or to the larger cone angle of
PMePhz (136’) relative to pyridine (132°).’.6 The Mo-Mo
distance, 2.128 A, is identical within experimental error in the
two complexes.
The terminal phosphine ligands are situated slightly off-axis
from the Mo-Mo vector, with the Mo-Mo-P angle equal to
166.15O (Figure 2). The disposition of the phosphine groups
appears to be dominated by external packing forces rather than
covalent interaction with the dimolybdenum units. A similar
situation was encountered in M O ~ ( O ~ C C F ~ )where
~ ( ~ the
~)~,
Mo-Mo-N angle was 171.Oo. The very weak bond between
Mo2-02
-04
-06
-08
2.131 (2)
2.115 (2)
2.093 (3)
2.128 (2)
av 2.117
Mo2-Mo 1-01
-03
-0 5
-07
MO1-Mo2-02
-04
-06
-08
91.34 (6)
90.01 (7)
91.07 (8)
92.83 (7)
90.88 (7)
92.95 (7)
92.14 (8)
89.52 (7)
av 91.34
w
(4) Cotton, F. A.; Norman, J. G. J. Am. Chem. Soc. 1972,94, 5697-5702.
(5) Pauling, L. “The Nature of the Chemical Bond”, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960; p 246.
( 6 ) Tolman, C. A. Chem. Rev. 1971, 77, 313-348.
Figure 1. O R T E ~drawing of axial M o ~ ( O ~ C C F J ~ ( P M
showing
~P~~)~
the labeling scheme. Ellipsoids represent 50% probability surfaces.
M O ~ ( O ~ C C and
F ~ ) the
~ axial ligands had been indicated
previously from N M R data, which suggested that extensive
or complete dissociation occurred in solution.’
1320 Inorganic Chemistry, Vol. 21, No. 4, 1982
Girolami and Andersen
a.
b.
Y
1662
1700
1600
Figure 2. View down the Mol-+Mo2 vector, showing disposition of
the PMePh, ligands with respect to the carboxylate planes.
Figure 3. Solid-state infrared spectra of the (a) axial and (b) equatorial
isomers of Mo2(O2CCF3),(PMePh,),.
Thus, it is established that two isomers of M0,(O,CCF,),(PMePh,), exist: one with a class I (axial) structure
and one with a class I1 (equatorial) structure.' This is not
surprising, since PMePh, lies very close to the class I/class
I1 boundary in our cone angle vs. basicity graph.]
We have investigated the conditions under which pure
samples of the two isomers may be obtained. The synthesis
of the axial isomer in diethyl ether is described in our original
paper.' It is important that the diethyl ether solution of
Mo2(02CCF3), and PMePh, be concentrated until it changes
color from yellow to orange and orange-yellow needles begin
to appear. Crystals of the axial diadduct appear when the
solution volume is ca. 5 mL. If the solution is not sufficiently
concentrated, the bis(diethy1 ether) adduct is obtained instead.,
No crystals of the equatorial isomer were ever isolated from
diethyl ether solution. The axial isomer may also be obtained
as the first crop of crystals from a 1:2 molar ratio of Mo2(02CCF3),:PMePh2 in toluene by cooling a saturated solution
to -10 OC. However, subsequent crops of crystals from this
solution are orange and consist entirely of the equatorial isomer
of M o ~ ( O , C C F ~ ) ~ ( P M ~ P(see
~ , )Experimental
,
Section for
details).
Each of the isomers gives an infrared spectrum characteristic
of its class (Figure 3). For the axial isomer, all the 0 2 C C F 3
groups are clearly bidentate,' as shown by the single asymmetric C 0 2 vibration at 1598 cm-'. In contrast, the equatorial
isomer exhibits strong bands at 1662 and 1574 cm-' due to
monodentate and bidentate 0 2 C C F 3groups, respectively.',*
This infrared spectrum is rather different from that reported
by Cotton and Lay for the equatorial isomer,, and it is likely
that their bulk sample was largely the axial isomer.
We have also investigated the 19Fand 31P(1HJ
N M R spectra
of the two compounds. Interestingly, pure samples of the axial
and equatorial isomers dissolve in CDC1, to give identical
N M R spectra have as their
spectra. At -50 O C , the 31P(1H)
major feature a broadened (vl = 100 Hz) peak at 6 -29 due
to the axial isomer of M o , ( O ~ ~ C F , ) , ( P M ~ P ~ ? )In, . addition,
~
small peaks occur at 6 +12, +8, and +7, resulting from various
equatorially substituted isomers. (See ref 1 for the solution
behavior of class I1 adducts.) The integrated intensities of the
peaks at 6 -29, +12, +8, and +7 are 100:23:18:19. Thus, the
major species in solution is the axial isomer. Upon warming
of the solution, the small peaks disappear, and the large peak
shifts until at 25 OC only a single resonance ( v l j 2 = 90 Hz)
at 6 -20 remains. It is apparent that a rapid equilibrium exists
in solution between the axial and various equatorial isomers
on the N M R time scale at room temperature.
The I9FN M R spectra mirror the 31P(1HJ
observations. At
-50 OC, a broad resonance ( Y ~ ,=~ 120 Hz) at 6 -72.3 is
observed due to the axial isomer. In addition, two sharp
resonances at 6 -71.4 and -74.5 result from bidentate and
monodentate 0 2 C C F 3groups of the equatorial isomers. The
area ratios of the peaks at 6 -72.3, -71.4, and -74.5 are
100:20:28, and again the axial isomer is seen to be the predominant species in solution. Upon warming of the solution
to 25 OC, the sharp peaks broaden and disappear, yielding a
single resonance (vl,, = 15 Hz) at 6 -72.5. These 19Fchemical
shifts are fully consistent with literature values for 0 2 C C F 3
ligand^.',^ However, the chemical shifts reported by Cotton
are different from these
and Lay for Mo,(O,CCF,)~(PM~P~,),
resuh2
,
(7) Only one other example of crystallographically characterized structural
isomers in dimolybdenum chemistry is known: Arenivar, J. D.; Mainz,
V. V.;Andersen, R. A.; Zalkin, A,; Ruben, H., submitted for publication.
(8) In addition, the equatorial isomer may be recognized by doubling of the
bands at 886, 775, and 690 cm-' in the axial isomer to give bands at
895, 886, 789, 774, 696, and 690 cm-'. Other features of the two
infrared spectra are superimposable.
Experimental Section
Axial Mo,(O,CCF~),(PM~P~~)~.
This isomer was prepared as
previously described, with the exception that the final volume is ca.
5 mL.' For additional comments, see the text of this paper.
Equatorial MO~(O~CCF~)~(PM~P~,)~
Methyldiphenylphosphine
(0.27 mL, 0.0015 mol) was added to Mo,(O,CCF,), (0.47 g, O.OO0 73
mol) in toluene (15 mL) under argon. After stirring for 1 h, the
solution was filtered and concentrated to ca. 5 mL. Cooling to -10
"C yielded 0.15 g (20%) of the axial isomer (identified by IR spectroscopy) as orange-yellow needles, mp 107 "C. The mother liquors
were recooled to -10 "C. Over a period of 1 day, a mixture of axial
and equatorial isomers of Mo2(O2CCF3),(PMePh2), was obtained.
Subsequent crops of the pure equatorial isomer crystallized over several
days at -10 "C as orange prisms, mp 116-1 18 OC. Total yield of
the equatorial isomer was 0.42 g, 55%. Anal. Calcd: C , 39.1; H,
2.51; P, 5.90. Found: C, 39.5; H, 2.69; P, 5.70.
X-ray Data. Large needles of the compound were cut to size and
mounted in thin-wall glass capillaries in air. The capillaries were then
flushed with nitrogen and flame-sealed.
Preliminary precession photographs yielded rough cell dimensions
and showed monoclinic Laue symmetry and systematic absences
consistent with space group P2,/c. A suitable crystal was then
transferred to a diffractometer, and standard peak search and au(9) King, R. B.; Kapoor, R. N. J. Organomer. Chem. 1968, 1 5 , 457-469.
Mitchell, C. M.; Stone, F. G. A. J . Chem. Soc., Dalton Trans. 1972,
102-107. Creswell, C. J.; Dobson, A.; Moore, D. S.; Robinson, S. D.
Inorg. Chem. 1979, 18, 2055-2059. Teramoto, K.; Sasaki, Y.; Migita,
K.; Iwaizumi, M.; Saito, K. Bull, Chem. SOC.Jpn. 1979, 52, 446-451.
Inorg. Chem. 1982, 21, 1321-1328
tomatic indexing procedures gave cell dimensions in agreement with
the photographic work. Least-squares refinement of the cell dimensions
with higher angle reflections yielded the values given in Table I.
Data were collected in one quadrant of reciprocal space (+h,+k,H)
by using measurement parameters given in Table I. Systematic
absences consistent only with space group P2,/c occurred at hOl, I
# 2n, and OkO, k # 2n. The measured intensities were reduced to
structure factor amplitudes and their esd's by correction for background, scan speed, and Lorentz and polarization effects. Corrections
for absorption or decay were unnecessary. Systematically absent
reflections were eliminated, and symmetry-equivalent reflections were
averaged to yield the set of unique reflections. Only those data with
I > 3 4 ) were used in the least-squares refinement.
The structure was solved by normal heavy-atom methods, hindered
slightly by near-special heavy-atom coordinates. The quantity minimized by the least-squares program was x.w(lFol - IFc1)2,and a p
factor of 0.03 for intense reflections was used throughout refinement.
The analytical forms for the scattering factor tables for the neutral
atoms were used, and all scattering factors were corrected for both
the real and imaginary components of anomalous dispersion.1°
(10) Cromer, D. T.; Waber, J. T. "International Tables for X-ray
Crystallography"; Kynoch Press: Birmingham, England, 1974; Vol. IV,
Tables 2.2B and 2.3.1.
1321
Hydrogen atoms were not located. In the final cycle, all atomic
positions and anisotropic thermal parameters were included in the
full-matrix least-squares refinement. Final refinement parameters
are given in Table I. All peaks in the final difference Fourier map
had intensities of less than 0.31 e A-3, and the largest were located
near fluorine atom positions or the phosphine methyl groups.
Acknowledgment. The crystal structure analysis was performed by Dr. F. J. Hollander, staff crystallographer a t the
University of California a t Berkeley X-ray Crystallographic
Facility (CHEXRAY). W e thank the NSF for departmental
grants used to purchase the NMR and X-ray spectrometers
employed in this work and Chevron for a fellowship (GSG).
W e also thank Professors E. L. Muetterties and K. N. Raymond for advice.
Registry No. Mo2(0,CCF&(PMePh2),, axial, 72453-45-3;
Mo,(O,,CCF,),(PM~P~,)~~
equatorial, 7603b-79-8; Mo2(O2CCF,),,
36608-07-8.
Supplementary Material Available: Figure 4 showing a stereopair
packing diagram of the unit cell contents, Figure 5 giving the numbering scheme for the 02CCF3 ligands, Table IV giving thermal
parameters, and Table V giving structure factors (27 pages). Ordering
information is given on any current masthead page.
Contribution from the Departments of Chemistry, The University of Texas of Austin, Austin, Texas 78712,
and University of Florida, Gainesville, Florida 32611
Structures of and Bonding in 7'-Cycloheptatrienylidene Complexes of Iron
PAUL E. RILEY, RAYMOND E. DAVIS,* NEIL T. ALLISON, and W. M. JONES*
Received July 11, 1980; Revised Manuscript Received June 8, 1981
The crystal structures of the PF6- salts of two 9'-cycloheptatrienylidene complexes of iron of the form [(q5-C5H5)(q1CHT)Fe(C0),]PF6, in which CHT = C7H6(1) or CllHs (7),have been determined by single-crystal X-ray diffraction
techniques with three-dimensional data gathered at -35 "C by counter methods. Yellow-orange crystals of 1 form as irregular
blocks in triclinic space group Pi with unit cell constants (at -35 "C) a = 7.981 (4) A, b = 14.378 (3) A, c = 7.133 (1)
A, a = 98.52 ( l ) " , /3 = 100.75 (l)", and y = 93.33 (1)". The calculated density of 1.728 g cm-,, with the assumption
of two formula weights of 1 per unit cell, agrees with the measured value of 1.70 g cm-,. Dark red crystals of 7 form as
irregular hexagonal prisms in space group Pi with unit cell constants (at -35 "C)a = 8.2086 (6) A, b = 15.238 (2) A,
c = 7.4361 (8) A, a = 90.509 (7)", /3 = 104.396 ( S ) " , and y = 94.676 (6)". The calculated density of 1.691 g cm-), with
the assumption of two formula weights of 7 per unit cell, agrees with the measured value of 1.68 g
Full-matrix
least-squares refinements of the structures have converged with conventional R indices (on 1q)of 0.057 and 0.041 for 1
and 7,respectively, with use of (in the same order) the 4042 and 4414 symmetry-independent reflections with Io/u(I,,)
> 2.0. Aside from the difference in the $-CHT rings, the molecular geometries of 1 and 7 are virtually identical, although
the shorter Fe-Cmb bond (by 0.017 A) in 1 suggests stronger Fe-carbene back-bonding in 1 than in 7. In addition, from
carbonyl stretching frequencies and IH and 13CNMR data, it is concluded that, despite the existence of these CHT ligands
in the carbene form, metal back-bonding with these 7' ligands is less important than in similar nonheteroatom-stabilized
carbene complexes such as [(75-CsHs)(q'-CH(Ph))Fe(CO)2](CF3S03).
It is noteworthy that the orientation of the CHT
rings is -90" from that observed in one Fe=CH2- and two Ta=CHR-containing complexes.
Introduction
Transition-metal-carbene complexes, or more specifically
methylene complexes, have long been considered as intermediates in olefin metathesis,14 cyclopropanation,4-' polymerization, and other reactions.6.* Though their existence, however
transitory, had thus often been inferred,@ the first species to
be isolated were the prototypical heteroatom-stabilized complexes MeC(OMe)W(C0)5, PhC(OMe)W(C0)5, and MeC(OMe)Cr(CO)5, which were prepared by Fischer'O and
characterized structurally by Mills." Since singlet carbene
carbon atoms (Cwb) possess a n unshared pair of electrons and
a n empty p orbital (i.e., sp2 hybridization), the bonding between metal and ligand is potentially similar to that between
a transition metal and a CO ligand (Le., synergistic).12
However, since the electrophilic Ccarb
atoms in Fischer-type
complexes are bonded directly to a heteroatom which possesses
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Mango, F. D.; Dvoretsky, I. J . Am. Chem. SOC.1966, 88, 1654.
Riley, P. E.; Capshew, C. E.; Pettit, R.; Davis, R. E. Inorg. Chem. 1978,
17, 408.
Green, M. L. H.; Ishaq, M.; Whiteley, R. N. J . Chem. SOC.A 1967,
1508; Cooper, J.; Green, M. L. H. J. Chem. Soc., Chem. Commun.
1974, 761.
Wong, W. K.; Tam, W.; Gladysz, J. A. J . Am. Chem. SOC.1979, 101,
5440.
*To whom correspondence should be addressed: R.E.D., The University
of Texas at Austin; W.M.J., University of Florida.
0020-1669/82/ 1321-1 321$01.25/0
Fischer, E. 0.; Maasbd, A. Angew. Chem. 1964, 76, 645; Angew.
Chem., Int. Ed. Engl. 1964, 3, 580.
Mills, 0. S.; Redhouse, A. D. Angew. Chem. 1965, 77, 1142; Angew.
Chem., Int. Ed. Engl. 1965, 4, 1082; J . Chem. SOC.A 1968, 642.
0 1982 American Chemical Society